1. Field of the Invention
The present invention relates to an impeller for an axial flow fan, and an axial flow fan using the impeller, and more particularly, to an impeller which maintains an air flow characteristic in a normal rotation direction without a significant deterioration in the air flow characteristic even in a case of rotating in a reverse direction, and an axial flow fan using the impeller.
2. Description of the Related Art
Axial flow fans have been used for blowing or cooling of electronic devices such as home appliances and information devices.
Electronic devices such as personal computers and copy machines include a number of electronic components accommodated in a relatively small casing. Therefore, heat generated from the electronic components stays in the casing, possibly resulting in destroying the electronic components. Thermal destruction causes a big problem for the device. For this reason, a ventilation hole is provided on the side wall or ceiling of the casing of the electronic device. The heat generated in the casing is discharged from the ventilation hole to the outside. Also, axial flow fans have been used as cooling means for electronic devices.
FIG. 7 is a front view showing a related-art axial flow fan.
The axial flow fan is disclosed in JP-A-H8-303391, and FIG. 8 is a cross-sectional view taken along a line B-B′ of a blade shown in FIG. 7. The blade shape of the axial flow fan shown in FIGS. 7 and 8 configures a forward swept blade. In order to increase an air flow, the blade shape is bent with respect to a rotation direction (normal rotation direction) such that a pressure surface side becomes a concave surface.
Some axial flow fan is rotatable in a reverse direction to change an air flow direction such that the axial flow fan can be used not only for blowing but also for exhaust. Since the related-art axial flow fan as shown in FIGS. 7 and 8 has the blade shape for increasing an air flow with respect to the normal rotation direction, in a case of rotating the axial flow fan in the reverse direction, the air flow characteristic is significantly deteriorated as compared to the case of the normal rotation direction.
Meanwhile, there is a known bi-directional axial blower which is rotatable in a normal direction and a reverse direction and is called as a jet fan for air ventilation of a tunnel or the like (see JP-A-2009-097430, for example). The jet fan is configured to have the same air flow characteristic even if an air flow direction is changed between the normal direction and the reverse direction. Therefore, it is possible to send air forward or backward in a tunnel according to the internal environment situation of the tunnel.
FIG. 9 is a cross-sectional view showing a blade of the axial blower of JP-A-2009-097430.
As shown in FIG. 9, the blade is S-shaped, and has a point symmetrical shape with respect to a point A (the center of the blade chord). The thickness of the blade has the maximum value h at the position of the point A, and is 8% to 14% with respect to the blade chord length L. The edge has a shape having a radius of curvature r of 0.25% to 0.35% with respect to the length L of the blade chord. At the position as the apex of warping, a distance X from a front end (or rear end) of the blade is about 10% with respect to the length L of the blade chord, and the height C of the warping at that position is about 2% with respect to the blade chord length. An axial flow fan having this blade shape has the same air flow characteristic in both of normal rotation and reverse rotation.
As in the axial blower disclosed in JP-2009-097430, if a blade shape is S-shaped and has a point symmetrical shape, even if the rotation direction is changed, the same air flow characteristic can be achieved. However, in this case, the air flow characteristic in the normal rotation direction is deteriorated as compared to the axial flow fan disclosed in JP-A-H8-303391. For this reason, in a case where a high air flow characteristic in the normal rotation direction is required, a blower as disclosed in JP-A-2009-097430 may not satisfy that requirement.